Apparatus for making laminated integrated circuit devices

ABSTRACT

An apparatus for producing laminated integrated circuit cubes includes a hermetically sealable oven chamber. At least one conductive heating device is used to heat the cube to a cure temperature. A vacuum source is connected to the oven chamber to remove gas from between the integrated circuit layers prior to curing. A press device applies pressure to the cube while it is heated under vacuum. A cooling gas is circulated through the oven chamber after curing. A cooling fluid can also be circulated through cooling channels in at least one wall of the oven chamber to cool the chamber.

BACKGROUND OF THE INVENTION

This invention relates generally to apparatus for making laminatedintegrated circuit devices (LICDs).

DESCRIPTION OF THE RELEVANT ART

The lamination of single layer integrated circuit chips into a multiplelayer device, sometimes referenced by the art as a "cube", is used topackage several of such chips in a convenient unit device. Such cubesare especially desirable where limited space is available, for example,on missiles and satellites, but they are also useful in other deviceswhere space saving is desirable. The manufacture of such cubes typicallyutilizes an adhesive such as a polyimide to adhere the single layerchips together. The adhesive must be cured with heat, and preferablyunder a vacuum to remove air and other gases from between the circuitlayers. Further, pressure is applied to the cube during curing to form acompact, tightly adhered structure.

SUMMARY OF THE INVENTION

The invention provides an apparatus for making cubes which utilizes acube lamination oven. The lamination oven is capable of beinghermetically sealed. A vacuum source removes air and other gases fromthe oven. At least one conductive heating device is provided to heat thecube. A press device is provided to apply a force to the cube. A coolinggas is used to cool the cube following the heat-curing step.

The device for conductively heating the cube is preferably a cubecontact heater assembly. The cube contact heater assembly preferably hasat least two heating elements, which are used to apply heat to the cubein at least two locations to provide for more even heating of the cube.Most preferably, upper and lower heating elements are provided in upperand lower heating plates. The plates are heated by the heating elementsand conduct heat to the cube.

The cube lamination oven is provided to apply vacuum to the cube whilethe cube is being heated by the heating elements. A vacuum will removeair and other gases from the space surrounding the cube. The ovenpreferably also provides structure for measurement of the temperature ofthe cube and of portions of the oven. The temperature-measuringstructure can be thermocouples or other suitable structure. The ovenalso preferably provides for a non-contact temperature measurement ofthe cube, such as infrared temperature sensing. The oven can bedimensioned to hold a plurality of contact heater assemblies, to permitthe fabrication of several cubes simultaneously. The oven has areclosable door, preferably hinged, which can be sealed to withstandvacuum to 10⁻³ torr or greater.

Cooling systems preferably are provided to cool the cube and the wallsof the oven during the cooling cycle. A source of cooling gas such asnitrogen is provided to flow gas into the oven and cool the cubes afterheating. The openings are sealed with special medium vacuum fittings.Heating cables are fed through the chamber, and also sealed with mediumvacuum fittings. Pressure relief structure is preferably provided withthe oven in case the pressure of the cooling gas in the oven shouldreach unacceptable levels.

A press device is provided to apply a force to the cube while it isheated in the vacuum chamber. The press device applies the force througha shaft or other force-applying member. Press devices can be providedwithin the oven, however, access ports are preferably provided forpermitting the introduction of mechanical force-applying members intothe oven. Sealing structure is provided to seal the oven chamber and topermit the passage of the force-applying member through the chamberwall. Biasing structure can be provided to hold the force-applyingmember against the vacuum force of the oven chamber, to prevent theforce-applying member from being pulled into the chamber before it isrequired. The force-applying member is preferably made of a materialwhich can withstand the force and will not transfer heat out of the ovenchamber. Air cylinders are preferably utilized to apply force to theforce-applying member. Electric cylinders or alternativeforce-generating structure is also possible.

The process is preferably controlled by a programmable logic controlleror any computer with sufficient processing capabilities. The heatingelements, press device, and cooling fluid flow controls can havepredetermined set point values, start and stop times, and ramp-up andramp-down rates which can be programmed into, and controlled by, theprogrammable logic controller.

BRIEF DESCRIPTION OF THE DRAWINGS

There are shown in the drawings embodiments which are presentlypreferred, it being understood, however, that the invention is notlimited to the precise arrangements and instrumentalities shown,wherein:

FIG. 1 is a front elevation of a cube laminating oven according to theinvention, with the door to the oven and the face plate over the pressdevices removed.

FIG. 2 is a cross section taken along line 2--2 in FIG. 1.

FIG. 3 is a perspective view, partially broken away, with the door tothe oven and the face plate in place.

FIG. 4 is a front elevation of an oven door according to one embodimentof the invention.

FIG. 5 is a top plan view, partially in phantom and partially brokenaway, of an oven chamber according to the invention.

FIG. 6 is a side elevation, partially broken away, of an oven chamberaccording to the invention.

FIG. 6(a) is a perspective view, partially in phantom, of a door hingeassembly.

FIG. 7 is an exploded perspective view of a cube contact heaterassembly.

FIG. 8 is a plan view of a cube contact heater assembly.

FIG. 9 is a perspective view of a cube contact heater assembly.

FIG. 10 is a side elevation of a cube contact heater assembly in a firstmode of operation.

FIG. 11 is a side elevation of a cube contact heater assembly in asecond mode of operation.

FIG. 12 is an exploded perspective of a portion of a press deviceaccording to the invention.

FIG. 13 is a side elevation, partially in cross section and partiallybroken away, of an assembled press device according to the invention.

FIG. 14 is a side elevation of a cube laminating station according tothe invention.

FIG. 15 is a rear perspective of a cube laminating station.

FIG. 16 is a cross section of an oven chamber in a first mode ofoperation.

FIG. 17 is a cross section of an oven chamber in a second mode ofoperation.

FIG. 18(a-b) is a flow chart of a process according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention provides a cube laminating oven assembly forsimultaneously applying conductive heat, vacuum, and a force to alaminated integrated circuit device, or "cube". A currently preferredoven assembly 30 is shown in FIGS. 1-3. The oven assembly 30 has an ovenchamber portion 34, at least one cube contact heating device 38, and atleast one press device 40. As shown in FIG. 1, many such devices can beincorporated into the oven assembly 30 such that many cubes can belaminated simultaneously. The components can be individually controlledsuch that each cube that is being laminated can have differentcharacteristics.

Cube Lamination Oven

The cube lamination oven of the invention includes the oven chamber 34,the heater assemblies 38, and the press devices 40. The press devices 40can be located within the vacuum oven chamber 34, but preferably areprovided outside of the oven chamber 34 where they can be enclosed by apress device housing 44. A press device actuator 47 is preferably alsoprovided. A cooling gas inlet 45 is provided to introduce cooling gasinto the oven, preferably through a tube 49 with openings 51 aimed atthe cube fixture. Electrical connections can be provided by a cable 48which connects to a high vacuum electrical connection 53, which can beany suitable high or medium vacuum electrical connection. The connectioncan terminate in a series of pins 55 inside the oven chamber 34 to whichelectrical connections for thermocouples and other electrical devicescan be connected. A vacuum outlet 52 is provided to remove air from theoven.

Peripheral devices can assist in the operation of the cube laminationoven. The vacuum outlet 52 can connect to a vacuum conduit 60 whichconnects to a vacuum source such as a vacuum pump 64 (FIGS. 14-15). Thevacuum pump is preferably capable of drawing a vacuum of 10⁻³ torr orgreater in the vacuum chamber 34.

The lamination oven 30 can also include various process sensing devices,including temperature sensors such as thermocouples and infrareddetectors. Pressure sensors, load cells, and flow meters can also beincluded.

A personal computer 70 and/or another control device with sufficientprocessing power, such as a programmable logic controller (PLC), can beprovided to permit the programming and feedback control of thelaminating process (FIGS. 14-15). Also, the personal computer 70 can beused to download process control settings to a programmable logiccontroller 72, which can then signal the appropriate apparatus throughsuitable control circuitry. The lamination oven 30 according to theinvention is compact, and can be provided on a table 74 or othersuitable support for the convenience of the operator.

Vacuum Oven Chamber

The vacuum oven chamber must withstand vacuum greater than about 10⁻³torr and temperatures greater than about 800° C. The vacuum oven chamberis therefore made according to known medium vacuum (to about 10⁻⁴ torr)fabrication techniques, and of a durable material such as stainlesssteel, preferably 3/4 inch stainless steel. The stainless steel isselected to prevent outgassing. The vacuum oven chamber 34 is alsodesigned according to known medium vacuum techniques to avoid pockets ofair being trapped inside of the chamber, which might then leak out underthe vacuum causing what is known as a "virtual leak". The oven chamber34 can be constructed in many shapes and dimensions, but preferably isrectangular with sides 33, 35, and 37, top 39, base 41, and door 43.

The door 43 (FIGS. 3-6(a)) is used to provide access to the chamber 34and to hermetically close the chamber 34 so as to withstand mediumvacuum of up to 10⁻³ torr or greater. The door 43 preferably has sealingstructure such as the O-ring 84 which seats against the front face 88 ofthe vacuum chamber 34 so as to tightly seal the chamber 34. Othersuitable sealing structure is possible.

The door 43 can be mounted to the chamber 34 in any suitable fashion. Inthe preferred embodiment shown, the door has hinge arms 90 which fitover support posts 100 that are mounted to the chamber 34. A hinge pin94 passes through a suitable opening in the hinge arms 90 and throughslots 111 in the support posts 100 to secure the door 43 to the vacuumchamber 34. A locking bolt 95 extends through an aperture in the supportposts 100 and is seated in the hinge pin 94 to vertically support thehinge pin 94.

A lock or latch of suitable design is also provided. The latch caninclude a clasp 104 and a loop 108, or can alternatively be of someother suitable design. A handle 109 is provided to lever the loop 108 tosecure the loop 108 to the clasp 104. The loop 108 and handle 109 arehinged to a bracket 119 that is mounted to the door 43.

According to a preferred embodiment of the invention, the door hingeand/or latch are constructed so as to release nitrogen or cooling gaspressure in the event that this pressure becomes too high. The slots 111in the support posts 100 are large enough to permit movement of thehinge pin 94, and thus the door 43. Springs 113 are connected to thehinge pin 94 and the rear side 33 of the chamber 34, as at eye bolts125. A rise in pressure within the oven chamber 34 will cause the door43 to move against the biasing of the springs 113 and away from its seatagainst the face 88 of the chamber 34 to release the pressure within thechamber 34. Also, the loop 108 can have a spring 117. The handle 109 andbracket 119 can slide over the loop 108, against the bias of spring 117,when there is a sufficient pressure in the oven chamber 34 to overcomethe force of the spring 117. This will permit the release of pressurefrom the oven chamber 34. Other pressure relief structure, such aspressure relief valves, is also possible.

The door 43 and base 41 preferably have structure for cooling the doorand base to remove heat that is generated by the conductive heatingwhich takes place within the oven chamber 34. Preferably, cooling waterchannels are either formed in the door and/or base, or a cooling waterconduit 121 can be fixed to the door and/or base. Water enters thechannel through a cooling water inlet 115 and exits through an outlet120. Cooling water can be supplied through an inlet conduit 124, and canbe returned through an outlet conduit 128. It is also possible toprovide cooling water flow channels to cool the sides 33, 35, and 37, ortop 39, of the oven chamber 34.

The oven chamber 34 can be fitted with temperature sensing devices suchas thermocouples. Structure is also preferably provided to facilitatethe non-contact temperature measurement of a cube within the ovenchamber. Windows 140 are preferably provided in the door 43 or otherwall of the vacuum chamber 34. The windows are sealed with suitablevacuum fittings. The windows are made of an infrared-transparentmaterial to permit direct non-contact temperature measurement of thecube using an infrared detector, such as the detector 141 shown in FIG.16. A window 140 can be provided in the door 43 to align with eachcontact heater assembly 38. A currently preferred material for thewindows is zinc selenide.

Cube Contact Heater Assembly

The cube contact heater assembly (FIGS. 7-9) preferably includes one ormore conductive heating elements which heat the cube. At least twoheating elements are preferably provided to apply conductive heat to thecube in different locations. This will help to prevent wide (more thanabout 5° C.) temperature differences, and the resulting stresses, in thecube. The heating elements may either contact the cube directly, orindirectly through one or more solid heat-conducting members. An upperheater 160 and a lower heater 164 are currently preferred. The upperheater 160 is shown in FIG. 7. Electrical power enters through cable178, and is connected to a heating element 180. The heating element 180is preferably provided in an enclosure 184. The heating element 180 mustbe capable of heating in a vacuum environment, without out-gassing, andshould preferably be flexible to permit coiling into the enclosure 1 84.The enclosure 184 must be made of a material that will not out-gas, suchas a low sulfur stainless steel, and should be capable of movementrelative to the cube. The element 180 is preferably wound within theenclosure 184 in a coil 185, in order to better distribute the heat. Athermocouple can be provided for sensing the temperature of the upperheater 160. Thermocouple wires can enter the enclosure 184 through thecable 178. The power wires and the thermocouple wires are connected tosuitable clips or other connections in a wall of the chamber 34 in asuitable medium vacuum electrical connection 179 (FIG. 2). The flexiblewires permit the upper heater 160 to move up and down.

The enclosure 184 preferably comprises two portions, a lower portion 188and an upper portion 190. These portions can be joined together bysuitable structures such as screws, and when joined create a pocket toreceived the heating element 180. The upper portion 190 has a roundedtop portion 194, that is preferably substantially semispherical, toprovide a "point" contact with the force-applying member of the pressdevice 40 that presses on it. This helps to alleviate alignmentproblems, and also reduces the conductive transmission of heat throughthe upper portion 190 and into the press device 40.

The lower heater 164 has a heating element 200. An electrical conduit202 has sealed wires to provide electric power to the heating element200. The conduit 202 is connected to a wall of the chamber 34 by asuitable vacuum connection 201. Thermocouple wires can also be providedin the lower heater 164 to sense the temperature of the lower heater.The element 200 is preferably enclosed by a base 206 and a cover 210.The base 206 has a depression 207, which with a cover 210 provides apocket to receive the heating element 200 which is preferably shaped asa coil. The cover 210 rests over the element 200, and can be secured tothe base 206 by suitable fastening structure such as screws 218 whichare received in apertures 220. The base 206 can be incorporated as partof a pedestal 224 which can conveniently be placed into and removed fromthe oven. The pedestal 224 is comprised of legs 228 which can help toinsulate the base 206 from the oven floor 41. The space 230 between thelegs 228 can be positioned over a positioning member 236 (FIG. 1), whichcan have four ninety-degree legs to form a "+" shape, that is providedon the floor 41 to properly position each contact heater assembly withinthe oven chamber 34. The spaces 230 between the legs 228 fit over thelegs of the positioning member 236 to provide secure positioning andengagement.

Structure is preferably provided for aligning the cubes on the contactheater assembly 38. Guide members 244 can be provided and have insidefaces 248 which act to position the cube directly under the press deviceand between the upper and lower heaters at the most desirable position.Inwardly inclined wedge surfaces 250 can be provided to direct the cubeinto position between the inside faces 248 as it is pushed onto thecontact heater assembly 38. The guide members 244 can be of any suitableconstruction, but preferably have lower depending flanges 264 which aresecured to the base 206 by suitable structure such as screws 270 whichpass through apertures 271 in the guide members 244 and engage apertures273 in the base 206. A rear guide member 274 with an inside face 275 canalso be provided to properly center the cube in the front-backdirection. The rear guide member can similarly be secured by a dependingflange 284 and screws 288. A switch 289 can be provided which iscontacted by the cube and will indicate to the computer controls thatthe heater assembly 38 has a cube in place.

Structure is preferably provided to mount the upper heater 160 so thatit is vertically movable over the base 206. Preferably, mounting posts300 are provided. Each mounting post 300 can have a mounting pin 302which is received by a mounting aperture 306 in the guide members 244.An upper socket portion 310 is adapted to receive a bolt 314. The bolt314 and washer 320 can be used to engage a suitable aperture 328 in theupper heater 160 to secure the upper heater to the contact heaterassembly. Springs 330 are used to bias the upper heater 160 upward so asto permit the insertion and removal of the cube fixture. The springs arepreferably provided over the socket portion 310 and cooperate to urgethe upper heater 160 away from the lower heater 164. The upper heater160 will move downward against the spring biasing when the press device40 pushes downward on the upper heater 160.

A cube fixture 332 is preferably provided for use in the oven assemblyto hold the layers of the circuit cube together while the cube is beinglaminated. The fixture can be of any suitable design for retaining thecircuit layers in stacked alignment within the heating assembly 38. Thefixture should conduct heat and either be open topped or have avertically movable cap to permit pressure from the press device 40 toact on the cube. The fixture preferably has a base and upwardly directedwalls which contain the single layer integrated circuit cube. A cap canslide over or fit within the walls so as to be movable verticallyrelative to the base. The cap will then adjust to the necessary heightfor the height of the cube.

The upper heater 160, when pressed downwardly by the press device 40,will contact the cube or a cap and transfer heat by conductive means tothe cube. Similarly, heat will be conducted from the lower heater 164through the base and to the cube. The cube fixture preferably permitsthe infrared viewing of the cube for purposes of temperaturemeasurement. Thermocouples, such as the thermocouple 339, should beeither in contact with the cube or as close to the cube as possible.Thermocouple fittings can be provided in the cube fixture.

Structure is preferably provided for securing the cube or the cubefixture in the contact heater assembly 38. The securing structure can beany suitable structure, such as the clamps shown in FIGS. 7 and 10-11.The clamps 344 can be hinged by screws 348 or the like, which can bereceived in apertures 350 in the guide members 244. The clamps 344 canhave inwardly directed locking arms 356. Inclined faces 360 are providedon the locking arms 356, such that when the inclined faces 360 arecontacted by the cube or the fixture the clamps are pushed outwardly andpivot about the screws 348 (FIG. 10). Counterweights 364 or othersuitable structure are provided to return the clamps 344 to the originalposition when the cube fixture is in place (FIG. 11). A pin 368 that canbe biased by a spring 380 can be provided on the rear guide member 274to bias the cube fixture against an inside face 370 of the locking arms356. This will position the fixture in the desired position each time itis placed into the contact heater assembly 38.

Vertical Press Device

The vertical press device 40 is designed to apply significant mechanicalforce to the cube while it is heated in an oven chamber 34. The vacuumin the chamber 34 can reach 10⁻³ torr or more and the temperature of thecube can reach 340°-360° C. It is possible to construct a mechanicalpress device for inclusion inside the oven chamber 34 which will becapable of operating under this vacuum and when subjected to thesetemperatures, however, it has been found preferable to construct a pressdevice outside of the oven chamber 34. The device is constructed in sucha manner as to prevent the loss of vacuum and to isolate as much aspossible the press device 40 from the elevated temperatures within theoven chamber 34.

The force generated by the press device can approach 100 lbs., and couldrequire more in special applications. The press device 40 consistsprincipally of a force-applying member 400 (FIGS. 12-13). Theforce-applying member 400 is preferably in the shape of an elongatedshaft, but could alternatively be provided in different shapes. Theshaft 400 extends through a suitable opening in the top wall 39 of theoven chamber 34 (FIG. 2). A fitting 410 loosely guides the shaft 400,and a flange 412 can secure the fitting in place over the opening. Asurrounding structural fitting 414 can be provided, if necessary, tolend additional mechanical support to the assembly. The fitting 410 canbe made of any suitable material capable of withstanding medium vacuumand high temperatures.

The shaft 400 must be tightly sealed in order to prevent the loss ofvacuum. To accomplish this, a seal is provided on the outside of theoven chamber 34. A flexible seal is preferably provided in the form of abellows or sheath. The sheath consists primarily of a flexible sheath424 of a hermetic material (FIG. 13). The sheath 424 is preferablyformed from a thin metal that can withstand medium vacuum, such asstainless steel. The sheath 424 is preferably hermetically attached tothe top wall 39 and to the shaft 400, to hermetically seal the shaft 400in the opening.

The sheath 424 can have a base plate 428 and an upper mounting plate432. The base plate 428 is mounted to the upper surface of the top wall39. The base plate 428 and the upper mounting plate 432 can havesuitable fastening structure such as apertures 436 to receive suitablefasteners such as screws 437. Alternatively, the sheath can be securedby other suitable fastening means. A central opening 430 extends throughthe upper mounting plate 432, the sheath 424, and the base plate 428 topermit the shaft 400 to extend into the oven chamber 34.

The shaft 400 can have an end plate 438 at an end distal to the ovenchamber 34. The space surrounding the shaft 400 can be completely sealedagainst the loss of vacuum through leakage around the shaft 400 bysuitable sealing structure such as o-ring 429 in the base plate 428 ando-ring 433 in upper mounting plate 432. These o-rings seal the baseplate 428 against the top 39 of the chamber 34, and seal the uppermounting plate 432 against the end plate 438. Screws 437 can engageapertures 439 in the end plate 438 to secure the upper mounting plate432 to the end plate 438 (FIG. 13).

Air and other gases may accumulate within the sheath 424 when the ovenis opened, since air can leak past the fitting 410. It is desirable toprovide structure for removing this gas when the vacuum is created inthe oven chamber 34. Channels 435 can be provided in the shaft 400 tofacilitate the withdrawal of air and gas from within the sheath 424 whenthe vacuum is applied to the oven chamber 34.

The shaft 400 is manipulated by a drive shaft 440 (FIG. 2). The driveshaft 440 is connected to the actuator 47, which can be pneumatic orelectric. In the preferred embodiment, the actuator is pneumatic andreceives power through a pressurized line 450 which connects to asuitable fitting 454. A piston 464 is connected to a cylinder adapter456. A fastener such as a bolt or screw 460 is used to connect thecylinder adapter 456 to a plunger 458. The plunger 458 has side walls459 and an inside top surface 461which rest around and over load cell470. The side walls 459 define openings in the front and rear of theplunger 458 to permit air to flow through the plunger over the load cell470. The load cell 470 can be selected from any suitable load cell forthis purpose. Preferably the load cell is of the compression type, witha linear voltage output, and a range that is preferably between 0-500lbs. A sensor wire 472 is provided to convey load cell signals to thecontrol system. The load cell is connected to a load cell plate 474. Anaperture 475 in the load cell plate 474 receives a portion 477 of theload cell 470 to secure the load cell in place. Surfaces 481 can beprovided on the load cell plate 474 and are engaged by the side walls459 of the plunger 458 to prevent rotation of the plunger 458. Thesurfaces 481 can be omitted if the pneumatic actuator is keyed toprevent rotation.

The load cell plate 474 abuts a biasing plate 480. The biasing plate 480has an elongated slot 486. Apertures 490 in the biasing plate 480receive shoulder bolts 494 or other suitable fastening structure. Theshoulder bolts 494 extend through the apertures 490 and engage suitablebiasing structure such as springs 498, 500, which can be separated bywashers 504. Threaded ends 510 of the shoulder bolts engage suitableopenings in the upper surface of the top wall 39 of the chamber 34 (FIG.13). The biasing plate 480 will be biased upwardly by the springs 498,500. Enlarged heads 514 of the shoulder bolts 494 secure the biasingplate 480 on the shoulder bolts 494.

A spacer plate 520 is attached to the load cell plate 474 by suitablestructure such as screws 524. An upwardly extending guide 530 includesapertures 534 to receive the screws 524. The guide 530 is dimensioned tofit into the slot 486 in the biasing plate 480. The spacer plate 520 canbe attached to the end plate 438 by suitable structure such as screws540 which pass through apertures 544 in the spacer plate 520 and engagethreaded apertures 548 in the end plate 438. The spacer plate 520 spacesthe end plate 438 from the load cell plate 474, and permits the biasingplate 480 to move freely over the guide 530 between the load cell plate474 and the end plate 438.

In operation, the biasing plate 480 will be urged by the springs 498,500 away from the top wall 39. The biasing plate 480 will contact theload cell plate 474. The load cell plate 474 is connected by the spacerplate 520 to the end plate 438 and the shaft 400, and the action of thesprings and biasing plate 480 will be to lift and retain the shaft 400in a position that is out of contact with the contact heater assembly38.

Downward movement of the drive shaft 440 and piston 464 will move theside walls 459 of the plunger 458 into contact with the biasing plate480. The biasing plate 480 is moved downwardly over the guide 530,against the biasing of the springs 498, 500, to a position between theend plate 438 and the load cell plate 474. The load cell 470 is thencontacted by the inside top surface 461 of the plunger 458, and the loadcell 470, load cell plate 474, and spacer plate 520 move in tandem tourge the end plate 438 and shaft 400 into contact with the heaterassembly 38. The load cell 470 will thereby not give false readingsresulting from the action of the springs 498, 500.

It can be seen that the shaft 400 will move upward and downward intandem with the sheath upon operation of the actuator 47. The load cell470 will provide an accurate reading of the force that is being appliedby the shaft 400. The springs 498, 500 acting on the plate 480 will actto lift the shaft out of the oven chamber 34 except when a downwardforce is applied by the actuator 47. It may be possible to avoid the useof springs 498, 500 if a double-acting actuator is provided, which willlift the shaft 400 out of contact with the heater assembly 38 inaddition to forcing the shaft 400 downward against the assembly.

Process Control

The process of the invention is preferably computer controlled by thecomputer 70, a programmable logic controller, or any other device withsufficient processing power and suitable interface circuitry. Threemajor programs are currently preferred. A first program is the operatorinterface and recipe creation software. A second is the trendingsoftware. The third is the process control program. These programs canbe any suitable software designed to control the process according tothe principles described herein.

The operator interface and recipe creation software can be provided bycommercially available software programs that create a graphical userinterface and a recipe creation facility. A suitable operator interfacesoftware is Winview or RSview manufactured by Rockwell Software ofMilwaukee, Wis. This software package runs on a personal computer in aWindows® environment and is customized by creating pictures withanimations, state changes, digital displays, buttons, lights and thelike and assigning them to memory addresses in a programmable logiccontroller (PLC) which runs the process control program. The operatorinterface software on the PC communicates to the PLC, if present, via astandard hardware communications interface such as RS-232.Product/process recipes are also created, selected and downloaded fromthe operator interface program to the PLC program. The product/processrecipe contains values such as temperature set points, temperature ramprates, force ramp rates, temperature offsets, dwell time, temperaturetrigger points, and the like.

The trending software can also be selected from commercially availablesoftware packages, such as RSTREND manufactured by Rockwell Software.This software is also preferably capable of running on a personalcomputer in a Windows® environment and is configured to create a stripchart record of the cube lamination process. The trending softwarerecords values such as temperature, force, vacuum and the like, and canprint out a chart record of these values for hard copy records. Thetrending software preferably also communicates to the process controlprogram via an appropriate interface such as RS-232 interface in similarfashion to the operator interface and recipe creation software.

The process control program (PCP) can be programmed in ladder logicprogramming language and runs on the programmable logic controller. ThePCP is designed to control the one or more contact heater assemblies inthe vacuum chamber 34 independently. The PCP also performs proportionalintegral derivative (PID) functions. Twelve such functions are currentlypreferred. Eight PID functions are for temperature control, two for eachof the four contact heater assemblies 38 in the preferred embodiment.Four PID functions are for force control, one for each press device 40.The PCP also accepts product recipe downloads from the operator PC, aswell as characterization value downloads. The PCP also contains separatesubroutines for independent temperature set point ramping and steadystate timing, as well as for temperature triggered force set pointramping.

The characterization values are required due to slight differencesbetween the contact heater assemblies such as thermocouple calibration,heater wattage, and heat transfer efficiency. The characterizationvalues are determined empirically from test runs that are conducted tomeasure these differences. The characterization values are subtracted oradded to some of the product/process recipe values such as temperatureset points and offsets. It may be possible to perform the processwithout some characterization values with good temperature and forcesensing, and with suitable feedback controls.

A substantially uniform temperature, with a variance of no more thanabout 5° C., preferably is maintained from top to bottom of the cube.The dual heater system of the preferred embodiment, utilizing the upperheater 160 and the lower heater 164, must be controlled due to thelarger heat sink of the lower heater 164. The upper heater 160 istherefore slaved to the lower heater 164, and is controlled to maintainan assigned temperature difference between the heaters. The PLC switchesboth the upper heater's slave relationship and slave offset fromfollowing the lower heater's temperature during the ramp to the firstcube temperature set point, to a constant value at the second cubetemperature set point. The switching of the slave relationships andoffsets is required due to the different temperature gradients whichdevelop in the contact heater assembly, fixture and cube during rampingconditions as opposed to steady state conditions.

In a preferred system, an operator interface is provided by an IBM® PCrunning PCDOS 6.3; Windows 3.1; and Winview or RSview (SupervisoryControl And Data Acquisition) operator interface and recipe creationapplication. A trend chart reporting function is performed by RSTrend.WinLinx provides a communications driver allowing RStrend and Winview tocommunicate to the PLC via RS-232. The preferred PLC is an Allen BradleySLC-500. The PLC runs a ladder diagram program which controls all toolfunctions. The PLC has special I/O modules. A digital input moduleinterfaces to switches in the oven which indicate that a cube fixture ispresent and that cooling gas and pneumatic pressure are present. Digitaloutput modules drive 24 VDC relays and solenoid valves on the oven whichturn heater power, vacuum, and water on and off. Analog input modulesreceive voltage or current signals from the load cells for forcesensing, the IR sensor for temperature sensing, and a vacuum gauge forvacuum sensing.

Analog output modules drive silicon controlled rectifiers (SCR) whichcontrol the proportional power to all heating elements. Thermocoupleinput modules receive signals from all the thermocouples for temperaturemeasurement.

The process flow chart shown in FIG. 18(a-b) provides a simplifiedsequence of a single station process according to the invention. Aplurality of station processes can be performed simultaneously. In block401, the characterization values are downloaded only when changes aremade to the contact heater assembly 38 such as thermocouple, heaterchanges and some mechanical changes such as materials, geometry, surfacefinish, or anything else that would affect heat transfer or require adifferent calibration. This is to account for slight differences in heattransfer efficiency. The PID parameters are similarly downloaded asshown in block 402. These are gain, integral and derivative values usedin the computer PID function. These process values are used to controlall heater assembly temperatures and press forces. A product/processrecipe is selected which remains in effect until a new recipe isdownloaded as shown in block 403.

The start is initiated at block 404 by operating a start button. A forceis applied to the chip stack by the press device in block 405. This isalso illustrated in FIG. 11 and 16. The vacuum valve is opened and thechamber is evacuated to a recipe vacuum set point, as shown in block406. In block 407, the cube stacking fixture is heated at temperatureramp rate number 1 to temperature set point number 1. When the cubetemperature reaches press force trigger temperature number 1, the pressforce is ramped to press force set point number 2, as shown in block408. Temperature set point number 1 is the temperature at which thetemperature ramp rate is changed to temperature ramp rate number 2, asshown by block 409. Heating continues at this new (usually slower) ramprate until the temperature of the cube reaches temperature set pointnumber 2. The upper heater is no longer slaved to the lower heater, butis instead given a constant value temperature set point, to allow thecube to attain a substantially uniform steady state temperature.

Temperature set point number 2 is the temperature at which the cube willbe cured, and the temperature is maintained at this value for a dwellperiod. A timer is started to measure the dwell period, as shown inblock 410. When the dwell period has expired, the power to the heatersis turned off, as shown by block 411. Cooling gas is allowed to flowinto the oven chamber 34 when the cube temperature reaches the nitrogentrigger temperature, as shown in block 412. The flow of nitrogen isbalanced with the vacuum pump to maintain a slight vacuum in the ovenchamber (illustrated by arrows in FIG. 17). Chilled water is circulatedthrough the chamber walls to speed up the cooling cycle when thetemperature falls below the chilled water trigger temperature, as shownin block 413.

The press force is negatively ramped to press force set point number 3when the temperature reaches press force trigger temperature number 2,as shown in block 414. When the cube temperature falls below press forcetrigger temperature number 3, the force of the press device is rampeddown to press force set point number 4 (usually 0), as shown in block415. The nitrogen gas and chilled water can be turned off at cubetemperature set point number 3, as shown in block 416. The process endsat block 417, whereupon the finished laminated integrated circuit cubecan be removed from the oven.

The particular process parameters that are used can be varied accordingto such factors as the type of adhesive that is used, the size andconstruction of the cube that is being fabricated, the materials makingup the cube, and the type and construction of the heating assembly andpress device. It is most desirable that the vacuum be sufficiently highthat substantially all gases be removed from between the circuit layersby the vacuum. The press force must be high enough to produce a compact,well-adhered cube, but must not be so high as to cause structural damageto the cube. The temperature must be high enough to effectively cure theadhesive, but must not be so high that damage to the cube and circuitsresults. The temperature will therefore be varied according to manyfactors, including the nature of the adhesive and the cube material.Uniform heating across the cube is desirable. The following example istherefore provided as an illustration of a currently preferred process,and it should be appreciated that other process parameters could bedesirable depending on the particular product and processcharacteristics.

The vacuum is selected to insure the removal of substantially all of thegas that may be between the circuit layers of the cube prior to heating,and can be adjusted as necessary. It is currently preferred that aminimum vacuum of 4×10⁻² torr be created in the oven chamber prior toheating. The pump will continue to draw down vacuum, eventually reachingbetween about 3×10⁻³ torr and about 7×10⁻³ torr.

A light force is preferably applied by the press device 40 at thebeginning of the process to cause the upper heater to move down andsecurely contact the cap of the fixture for heating. A force of 1-20 lbsis preferred for press force set point number 1, and is achieved atforce ramp rate number 1, preferably about 1 lb./sec. The conductiveheating then begins so as to raise the temperature of the cube at atemperature ramp rate. The ramp is selected from a recipe, but isinitially determined empirically. A temperature ramp rate that is toohigh will cause stresses in the cube. A ramp rate that is too low willneedlessly lengthen the process.

Preferably, an initial temperature ramp rate and a second temperatureramp rate are used. A reduced second temperature ramp rate can reducethe temperature overshoot typical in an underdamped control system. Aninitial temperature ramp rate of about 8° C./min is preferred, andinitial temperature ramp rates of between about 1° C./min and about 12°C./min are preferably used. The heating continues at the initialtemperature ramp rate until the temperature of the cube reaches cubetemperature set point number 1, about 340° C. The heating then continuesat a slower second temperature ramp rate of about 0.5° C./min until thecube reaches cube temperature set point number 2, currently about 345°C.

In one recipe, the slave relationship between the upper heater and thelower heater sets the upper heater to be 125° C. above the lower heatertemperature From cube temperature set point number 1through the end ofthe dwell period, the upper heater temperature is adjusted to be 437° C.higher than cube temperature set point number 2.

The initial force is maintained until the temperature reaches an initialpress force trigger temperature. Heating the cube under a light forceand under vacuum will give gas trapped between the chip layers time toescape. Higher initial forces would make this escape more difficult.This press force trigger temperature is selected from the recipe, and isinitially determined empirically. A currently preferred press forcetrigger temperature number 1 is about 250° C. The force applied to thecube by the press device 40 is then ramped up to press force set pointnumber 2. The press force ramp rate number 2 is currently about 5lb/sec, and press force set point number 2 is about 94 lbs. The pressforce set points can be any between about 0 lb. and about 100 lb. Higherforces can be attained in particular circumstances.

The cube temperature set point number 2 is maintained for a period oftime that is sufficient to permit the curing of the adhesive, referredto as the dwell time. The dwell time that is currently preferred isabout 30 minutes, but could change with different adhesives or differentcube constructions. Press force set point number 2 is maintained untilpress force trigger temperature number 2 is reached.

The completion of the temperature dwell cycle is followed by a coolingcycle. The nitrogen trigger temperature and chilled water triggertemperature are currently about 340° C. The cooling rate is determinedby the flow of cooling gas into and out of the chamber. A currentlypreferred cooling rate is about -10° C./min. Press force triggertemperature number 2 is provided to begin the reduction of the pressforce. Press force trigger temperature number 2 is currently about 200°C. The press force is then reduced, at press force ramp rate number 3,about -5 lb/sec, to press force set point number 3, between about 1-20lbs. When press force trigger temperature number 3 is reached, about101° C., the press force is ramped to press force set point number 4,usually 0 lbs, at press force ramp rate number 4, about -5 lb/sec. Atthe end of the process, all fixture temperatures are less than or equalto cube temperature set point number 3. The time for the total processis currently about 120 min to about 150 min.

The process is preferably controlled using suitable feedback controlmethods. Closed loop control methods, such as Proportional IntegralDerivative (PID) methods, are currently preferred.

Although the invention has been particularly described for makinglaminated integrated circuit cubes, it will be apparent to one skilledin the art that the invention has utility for heat-curing otherworkpieces where it is desirable to remove air and other gases frombetween portions of the workpiece and to tightly adhere such portionstogether. This invention can be provided in other specific embodimentswithout departing from the spirit or essential attributes thereof, andaccordingly, reference should be had to the following claims, ratherthan the foregoing specification, as indicating the scope of theinvention.

We claim:
 1. An apparatus for producing laminated integrated circuit cubes, comprising:a hermetically sealable oven chamber; a mechanical press device capable of applying pressure to the cube when the cube is inside the chamber; at least two conductive heaters for applying conductive heat to at least two portions of said cube, said heaters including an upper heater and a lower heater, each of said heaters comprising a heating element and a cover, said upper heater having structure for contacting said press device, said structure comprising a substantially semispherical surface; and, means for creating a vacuum in the chamber.
 2. The apparatus of claim 1, wherein at least the upper heater is movable relative to the cube.
 3. The apparatus of claim 1, wherein the lower heater is provided in a heating assembly, said heating assembly further comprising structure for fixing the cube in a desired position.
 4. The apparatus of claim 3, when said fixing structure includes retractable clamp structure.
 5. The apparatus of claim 1, further comprising structure for introducing a cooling gas into said oven chamber.
 6. The apparatus of claim 1, further comprising structure for determining the temperature of said cube.
 7. The apparatus of claim 6, wherein said structure comprises thermocouples.
 8. The apparatus of claim 6, further comprising structure for taking infrared heat measurements of said cubes.
 9. The apparatus of claim 1, wherein said oven chamber further comprises infrared-transparent windows for taking infrared measurements of cubes in said oven chamber.
 10. The apparatus of claim 1, wherein said windows are provided in a door to said oven chamber.
 11. The apparatus of claim 1, further comprising structure for cooling said oven chamber, said structure comprising cooling fluid channels.
 12. The apparatus of claim 1, further comprising structure for positioning said cube relative to at least one of said conductive heater and said mechanical press device.
 13. The apparatus of claim 1, further comprising a programmable controller, said programmable controller being adapted to control at least one of said conductive heating elements, press device, and a cooling gas.
 14. The apparatus of claim 1, wherein said press device comprises a shaft movably mounted through an opening in said enclosure, and comprising hermetic sealing structure about said opening and said shaft. 